The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Engines emit exhaust gas that includes carbon dioxide, water, carbon monoxide (CO), nitrogen oxides (NOx), unburned hydrocarbons (HC), and other compounds. Exhaust systems typically include a catalyst that reduces the levels of CO, NOx, and HC in the exhaust gas by chemically converting these gases into carbon dioxide, nitrogen, and water. The catalyst reduces the levels of the gases by catalyzing a reaction between the gases and oxygen. The catalyst stores oxygen when operating under lean fuel conditions and releases oxygen when operating under rich fuel conditions.
A vehicle may operate in a fuel cutoff mode during which fueling to the cylinders is disabled. The fuel cutoff mode may occur when the vehicle is decelerating with no throttle input from a driver and the engine is acting as a brake. During the fuel cutoff mode, the engine pumps air through the exhaust system rather than exhaust gas, resulting in delivery of excess oxygen to the catalyst. The catalyst stores the oxygen until a maximum oxygen storage amount is reached. When the maximum oxygen storage amount is exceeded, the ability of the catalyst to convert NOx emissions may be substantially reduced, resulting in NOx breakthrough. Therefore, when fueling resumes, an increased amount of fueling is delivered to the engine to cause rich fuel conditions. The rich fuel conditions decrease the amount of oxygen and improves NOx conversion.